SAR comparison between CASL and pCASL at high magnetic field and evaluation of the benefit of a dedicated labeling coil.

PURPOSE: To investigate the heating induced by (pseudo)-continuous arterial spin labeling ((p)CASL) sequences in vivo at 9.4T and to evaluate the benefit of a dedicated labeling coil. METHODS: Temperature was measured continuously in the brain, neck, and rectum of 9 rats with fiber-optic temperature probes while running pCASL-EPI and CASL-EPI sequences, with labeling B1 amplitudes (B1ave ) of 3, 5, and 7 µT and using a dedicated labeling RF coil or a volume coil. From the temperature time courses, the corresponding specific absorption rate (SAR) was computed. A trade-off between SAR and labeling quality was determined based on measured inversion efficiencies. RESULTS: ASL experiments with standard parameters (B1ave = 5 µT, Tacq = 4 min, labeling with volume coil) lead to a brain temperature increase due to RF of 0.72 ± 0.46 K for pCASL and 0.25 ± 0.17 K for CASL. Using a dedicated labeling coil reduced the RF-induced SAR by a factor of 10 in the brain and a factor of 2 in the neck. Besides SAR due to RF, heat from the coil decoupling circuits produced significant temperature increases. When labeling with a dedicated coil, this mechanism was the dominant source of brain heating. At equivalent RF-SAR, CASL provided slightly superior label efficiency to pCASL and is therefore the preferred sequence when an ASL coil is available. CONCLUSION: B1ave = 4-5 µT provided a good compromise between label efficiency and SAR, both for pCASL and CASL. The sensitivity of animals to heating should be taken into account when optimizing preclinical ASL protocols and may require reducing scan duration or lowering B1ave .

Interpulse phase corrections for unbalanced pseudo-continuous arterial spin labeling at high magnetic field.

PURPOSE: To evaluate a prescan-based radiofrequency phase-correction strategy for unbalanced pseudo-continuous arterial spin labeling (pCASL) at 9.4 T in vivo and to test its robustness toward suboptimal shim conditions. METHODS: Label and control interpulse phases were optimized separately by means of two prescans in rats. The mean perfusion as well as the interhemispherical symmetry were measured for several phase combinations (optimized versus theoretical phases) to evaluate the correction quality. Interpulse phases were also optimized under degraded shim conditions (i.e., up to four times the study shim values) to test the strategy's robustness. RESULTS: For all tested shim conditions, the full arterial spin labeling (ASL) signal could be restored. Without any correction, the relative ASL signal was 1.4 ± 1.7%. It increased to 3.6 ± 1.4% with an optimized label phase and to 5.3 ± 1.2% with optimized label and control phases. Moreover, asymmetry between brain hemispheres, which could be as high as 100% without phase optimization, was dramatically reduced to 1 ± 3% when applying optimized label and control phases. CONCLUSIONS: Pseudo-continuous ASL at high magnetic field is very sensitive to shim conditions. Label and control radiofrequency phase optimization based on prescans robustly maximizes the ASL signal obtained with unbalanced pCASL and minimizes the asymmetry between hemispheres. Magn Reson Med 79:1314-1324, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Impact of tissue T1 on perfusion measurement with arterial spin labeling.

PURPOSE: Arterial spin labeling (ASL) may provide quantitative maps of cerebral blood flow (CBF). Because labeled water exchanges with tissue water, this study evaluates the influence of tissue T1 on CBF quantification using ASL. METHODS: To locally modify T1 , a low dose of manganese (Mn) was intracerebrally injected in one hemisphere of 19 rats (cortex or striatum). Tissue T1 and CBF were mapped using inversion recovery and continuous ASL experiments at 4.7T. RESULTS: Mn reduced the tissue T1 by more than 30% but had little impact on other tissue properties as assessed via dynamic susceptibility and diffusion MRI. Using a single-compartment model, the use of a single tissue T1 value yielded a mean relative ipsilateral (Mn-injected) to contralateral (noninjected) CBF difference of -34% in cortex and -22% in striatum tissue. With a T1 map, these values became -7% and +8%, respectively. CONCLUSION: A low dose of Mn reduces the tissue T1 without modifying CBF. Heterogeneous T1 impacts the ASL estimate of CBF in a region-dependent way. In animals, and when T1 modifications exceed the accuracy with which the tissue T1 can be determined, an estimate of tissue T1 should be obtained when quantifying CBF with an ASL technique. Magn Reson Med 77:1656-1664, 2017. © 2016 International Society for Magnetic Resonance in Medicine.